It's the basic 'flaw' of Democracy: Since the majority of the people are old, they will vote in the interest of old people.
That's why the pension system will never be fixed.
The majority of people being old is a relatively modern problem. One could even argue that it is a problem specific to cultures that favor low or negative birthrates.
The list of countries with no low or negative birthrates is very short (mostly heavily underdeveloped countries). That's why the Gerontocracy applies nearly everywhere. I doubt it was favoured, but it's result is the shift of incentives from long term (young people) to short term (old people). Democracy fails here.
No cultures directly favor low or negative birthrates.
But we've known since the 1800s that it follows from female education (and this seems independent of culture, it was first observed in France, but you can see the same trend in any African country, or even Iran), which is favored.
France went through a demographic transition 100+ years earlier than all other European countries and then had a population boom in the 50s and 60s. I doubt an average French women was less educated in 1960 than in the mid 1800s.
I don’t think it’s that straightforward, material conditions in combination with massively lower mortality in all age groups and a shift in social values must be playing a significant part
Are we calling 40s old now? There are 2 countries where the median age is above 50 [0] and the US is 38.9. I suppose under 18s aren't in the voting pool but if a country has a median age of 50 they probably don't have that many under-18s running around. And old people don't vote as a totally unified a block, it'd be like saying countries are run for women because the median voter is a woman.
I'd suggest the main issue is that the world is so complicated that the younger voters just don't know what to organise and vote for. In the US in particular, they seem to basically be running an experiment every single election to try and figure out who they need to vote for to get some sane economic policy and stop getting involved in stupid wars. No success so far but you have to admire the process. The only people not getting the message are the people paid off to ignore it.